chore: import upstream snapshot with attribution

3rd_party/flatbuffers/tests/union_vector/BookReader.java

@@ -0,0 +1,21 @@
+// automatically generated by the FlatBuffers compiler, do not modify
+
+import java.nio.*;
+import java.lang.*;
+import java.util.*;
+import com.google.flatbuffers.*;
+
+@SuppressWarnings("unused")
+public final class BookReader extends Struct {
+  public void __init(int _i, ByteBuffer _bb) { bb_pos = _i; bb = _bb; }
+  public BookReader __assign(int _i, ByteBuffer _bb) { __init(_i, _bb); return this; }
+
+  public int booksRead() { return bb.getInt(bb_pos + 0); }
+
+  public static int createBookReader(FlatBufferBuilder builder, int booksRead) {
+    builder.prep(4, 4);
+    builder.putInt(booksRead);
+    return builder.offset();
+  }
+}
+